"what makes Japan's transit card system (IC cards) so unique compared to the West"
Actually other western transit system cards are similar to Japan's. For example in Paris the transit passes (Navigo cards) are stored-value systems and hold a record of the last few scans (three I think). You can also read them with any NFC smartphone and see what's stored inside. The tap at the entrance of a transit vehicle is near instant as the reader doesn't need to interact with a backend since everything is stored on the card itself.
Actually other western transit system cards are similar to Japan's. For example in Paris the transit passes (Navigo cards) are stored-value systems and hold a record of the last few scans (three I think). You can also read them with any NFC smartphone and see what's stored inside. The tap at the entrance of a transit vehicle is near instant as the reader doesn't need to interact with a backend since everything is stored on the card itself.